"Auckland's population predicted to grow by more than 700,000 people over the next 30 years, the CRL will play an important role in getting people in and out of the city with ease."
Building the country's first underground rail system was a "massive" project, he said, and the deal was a milestone.
Finance Minister Steven Joyce said the partnership was "crucial" and would ensure a high-quality result.
"This is a complex and critical piece of infrastructure that will unlock major development opportunities across central Auckland."
Once complete, the 3.4km double-track underground rail line will run from Britomart station through the CBD, connecting with the existing western line at Mt Eden station.